An image sensor comprises a matrix of pixels organized into rows and columns. The pixels of one and the same column are linked to a column conductor which is itself linked to a reading circuit, to convert into digital the voltage level applied to the column conductor by a pixel selected for reading. This voltage level represents the light level received by the pixel. This conversion is carried out by a ramp-type analogue-digital converter, which comprises a comparator for comparing the voltage level to be converted with a linear voltage ramp, and a counter which counts at high frequency from the instant the ramp starts until the comparator toggles. The content of the counter at the instant of toggling represents a numerical value of the voltage level to be converted.
The pixel mainly comprises a photodiode and MOS transistors, for example four transistors, which make it possible to drive the reading of the charge generated by the light in the photodiode. The pixels with four transistors operate by transferring charge from the photodiode to a capacitive storage node previously initialized to a reinitialization potential; the reading of the pixel in general comprises the carrying over onto the column conductor of the reinitialization potential and then of the potential of the storage node representative of the light level received by the pixel.
In a known manner, to circumvent the noise associated with the capacitive storage node of the pixel, which is correlated noise that is encountered equally well with regard to the reinitialization level of the storage node as with regard to the useful level after transfer of the charge received, it is possible to perform an analogue-digital conversion of each of the two levels, reinitialization level and useful level, and a subtraction is performed between the two numerical values, thus eliminating this correlated-noise component. It is also possible to do an analogue subtraction of the voltages before converting the difference. In this case, two sampling capacitors are used to sample the electrical signal provided by the column, one after reinitialization and the other after charge transfer, before performing a differential conversion.
The reading circuit is dimensioned as a function of the dynamic range of the sensor, that is to say of the span of illuminations that it is desired to measure. Notably the duration of the ramp is determined so as to allow the conversion of the highest illumination level that it is desired to measure, having regard to the counting frequency, the number of bits of the counter, and the desired conversion rate.
The dynamic range of these image sensors is in practice limited towards the low luminance levels, by the various sources of noise of the conversion chain, comprising the transistor mounted as follower in the pixel, so as to copy the potential of the storage node onto the column conductor, the current source common to all the columns, the large-gain amplifier forming the comparator etc. This chain noise is completely random and independent of the level of the signal to be measured. It constitutes an impediment to the ability to increase the dynamic range of sensors towards low illumination levels.
The invention is more particularly concerned with reading circuits which directly perform an analogue-digital conversion of each of the two voltage levels provided by the column, and the subtraction between the two numerical values obtained. In this context in fact, theoretically it is known how to reduce in a root of n ratio, the amount of Gaussian noise present in a signal, by performing n conversions of this signal and by averaging these n conversions.
But performing n conversions multiplies the time necessary for reading a pixel, which is fixed mainly by the nominal duration of the ramp, determined so as to obtain a wide dynamic range and good resolution for these sensors.